This invention relates to dual coated self cleaning glazing sheets and to laminates and multiple glazing units having coatings on at least two surfaces thereof at least one of which is a self cleaning coating.
Self cleaning glazing sheets having a coating which is hydrophilic and photocatalytically active have become articles of commerce in recent years. The coatings generally comprise a metal oxide such as titanium oxide, iron oxides, silver oxides, copper oxides, tungsten oxides, zinc oxides and zinc/tin oxides. The oxide which is most commonly used is titanium dioxide. Self cleaning glazing sheets having a titanium dioxide coating on one surface thereof are disclosed in EP 901991, WO 97/07069, WO 97/10186, WO 98/06675 and WO 98/41480.
Other functional coatings which impart different properties to the coated glass sheet are also known. Thin coatings comprising transparent metal and metal oxide layers singly or in multiple layers are used to regulate the transmission of radiation through the glass. Low-E coatings allow solar energy transmission through the glass but reflect longer wavelength radiation and thereby conserve heat within a building. Solar control coatings reflect or absorb solar energy and thereby reduce the amount of heat entering a building or vehicle. These coatings are normally applied to one surface of a glass sheet. When the coated glass is incorporated into a laminated window or a multiple glazing unit the coated surface is normally positioned on an interior surface so as to avoid damage to the coating.
In contrast the coated surface of a self cleaning glazing sheet must be positioned on an exterior face of a glazing. The active coating may face the interior of a building or a vehicle. However more commonly the active coating faces the outside of the building or vehicle where it may be washed by rain water.
The application of a functional coating to the reverse side of a glass sheet having a self cleaning coating upon its front surface presents an opportunity to produce dual coated glasses having a coating on both sides of the glass sheet which might exhibit an advantageous combination of properties. The production of such a dual coated glass sheet poses a number of problems. Firstly the glass sheets to be coated are normally produced using the float glass process. Production of a dual coated sheet requires the deposition of a coating upon the lower or tin side of the glass as well as on its upper or air side. Secondly the coatings need to be applied with care to ensure that the entire surface of the sheet is coated whilst not contaminating the opposite face and thirdly the dual coated sheet must have acceptable properties including optical properties. The application of a functional coating such as a heat reflecting coating to the reverse side of a glass sheet having a self cleaning coating on its front side can cause the dual coated product to have an unacceptable coloration.